Method of operating a lidar system for detection of gas

ABSTRACT

A lidar system for detection of a gas comprises an optical transceiver for transmitting and receiving optical radiation. A method of operating the system comprises performing spatially scanned sensing measurements of the gas across a system field of view, and analyzing the sensing measurements to determine the presence and location of excess of the gas in the system field of view. Based on the determined location, an adjusted system field of view is determined and spatially scanned sensing measurements of the gas are performed across the adjusted system field of view to obtain sensing measurements at higher spatial resolution.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 17/806,039, filed Jun. 8, 2022, which claims the benefit ofpriority of U.S. Provisional Patent Application No. 63/202,375, filedJun. 8, 2021, entitled METHOD OF SCANNING IN A LASER LIDAR SYSTEM, allof which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method of operating a lidar systemfor detection of a gas, for example a particular gas that might bepresent in an environment in a larger concentration than normal.

BACKGROUND OF THE INVENTION

An example of a lidar system for gas detection is described in UK PatentApplication Publication No. GB2586075A by J. Titchener and X. Ai,entitled “Rapidly tunable diode lidar” and published 3 Feb. 2021(“GB2586075A”), which uses a tuned laser wavelength to detect a gas.

Some of the methods and systems described below are directed toimproving the resolution of gas detection. Some of the methods andsystems described below may solve other problems.

SUMMARY OF THE INVENTION

There is provided in the following a method of operating a lidar systemfor detection of a gas, wherein the system comprises an opticaltransceiver for transmitting and receiving optical radiation. The methodcomprises performing spatially scanned sensing measurements of the gasacross a system field of view; analyzing the sensing measurements todetermine the presence and location of excess of the gas in the systemfield of view; based on the determined location, determining an adjustedsystem field of view and performing spatially scanned sensingmeasurements of the gas across the adjusted system field of view toobtain sensing measurements at higher spatial resolution.

There is also provided a system configured to perform the foregoingmethod. The method may be implemented by way of a computer readablemedium.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the invention will now be described by way of exampleonly and with reference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of a lidar gas detection system in whichany of the methods to be described here may be implemented;

FIGS. 2A-C are a series of diagrams illustrating how dual prisms may beused to steer an optical beam;

FIGS. 3A and 3B are respectively a perspective and a cut-away viewillustrating how dual rotating prisms may be incorporated into amechanical design;

FIGS. 4A and 4B show examples of how an optical beam can be made to scanaround a field of view in a variety of different patterns by using dualrotating prisms;

FIGS. 5A and 5B show examples of how an optical beam can be made to scanaround in circles with different sizes by using synchronized rotation ofdual prisms;

FIGS. 6A-C show an example of how an optical beam can be made to scanaround a reduced size field of view by superimposing a sinusoidaloscillation of the relative prism-to-prism angle onto a synchronizedrotation of dual prisms;

FIGS. 7A-C show an example of how an optical beam can be made to scanaround a field of view in a spiral pattern by reducing the rate of thesuperimposed oscillation of the prism-to-prism angle with respect to therate of the synchronous rotation of the prism pair;

FIGS. 8A-C show an example of how an optical beam being scanned in aspiral pattern can be made to maintain beam scanning speed by increasingthe synchronous rotation rate of the prism pair as the scan spiralstowards the center of the field of view;

FIGS. 9A-C show a series of images obtained with a lidar gas detectionsystem with different fields of view created by a dual rotating prismscanner;

FIG. 10 is a schematic diagram of a gas lidar beam pointing and scanningsystem;

FIG. 11 is a flowchart of a method of operating a lidar gas detectionsystem.

DETAILED DESCRIPTION OF ONE OR MORE EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications in the illustrated device, and such further applicationsof the principles of the invention as illustrated therein beingcontemplated as would normally occur to one skilled in the art to whichthe invention relates. At least one embodiment of the present inventionwill be described and shown, and this application may show and/ordescribe other embodiments of the present invention, and further permitsthe reasonable and logical inference of still other embodiments as wouldbe understood by persons of ordinary skill in the art.

It is understood that any reference to “the invention” is a reference toan embodiment of a family of inventions, with no single embodimentincluding an apparatus, process, or composition that should be includedin all embodiments, unless otherwise stated. Further, although there maybe discussion with regards to “advantages” provided by some embodimentsof the present invention, it is understood that yet other embodimentsmay not include those same advantages, or may include yet differentadvantages. Any advantages described herein are not to be construed aslimiting to any of the claims. The usage of words indicating preference,such as “various embodiments” or “preferably,” refers to features andaspects that are present in at least one embodiment, but which areoptional for some embodiments, it therefore being understood that use ofthe words “preferred” and/or “preferably” (and/or inflections thereof)implies the term “optional.”

FIG. 1 is a is a schematic diagram of a lidar gas detection system inwhich any of the methods to be described here may be implemented. Thesystem is described in more detail in GB2586075A, which is incorporatedherein by reference.

As shown in FIG. 1 , the gas detection system 1 is a gas detectiondevice configured to detect the presence or concentration of at leastone gas 2.

The gas detection system 1 includes a laser device 4 operable to outputfirst output radiation 6 having a continuous wave output. The gasdetection system 1 includes a control element 8 operable to tune a firstemission wavelength 9 of the first output radiation 6 continuouslywithin a first wavelength spectrum 10.

The control element 8 is operable to continuously tune the firstemission 30 wavelength 9 within the first wavelength spectrum 10 and toperform multiple scans within the first wavelength spectrum 10. In thisarrangement, the gas detection system 1 is operable to continuously varythe first wavelength spectrum 10, such that the emission wavelength 9varies continuously over time. A gas may be detected in a number of waysas known to those skilled in the art, for example based on itscharacteristic transmission or absorption spectrum.

As shown in FIGS. 1 and 2A-C, the gas detection system 1 includes amodulator 14 operable to apply a first output modulation to the firstoutput radiation 6.

The gas detection system 1 includes an optical system 26 operable totransmit the first output radiation 6 towards a first target location 18and to collect/receive scattered radiation 20, the scattered radiation20 having been at least partially modified by the gas 2 present in thefirst target location 18.

The gas detection system 1 includes a detector 22 configured to receivethe scattered radiation 20 and a processing element 24 operable toprocess the received scattered radiation 20.

In the embodiments illustrated and described here, the gas detectionsystem 1 is configured to detect the presence, or concentration, of thegas 2 in the atmosphere.

In the embodiments illustrated and described here, the gas detectionsystem 1 is configured to detect the presence, or concentration, of thegas 2 when located remote from the gas detection system 1, at a distanceof up to approximately 100 meters. However, it should be appreciatedthat the gas detection system 1 could be configured to detect the gas 2at other distances. For example, the gas detection system 1 could bemounted to a satellite and configured to operate at distances of up to100 km or more. Furthermore, the gas detection system could beconfigured to detect gas located within the gas detection system 1.

In an example implementation the gas detection system 1 is configured todetect methane (CH4), although the gas detection system 1 could beconfigured to detect one or more gases.

In use, the gas detection system 1 is operable to perform one or morescans of a target area or location 18. In this arrangement, the gasdetection system 1 is operable to obtain spatially scanned sensingmeasurements of the gas across a system field of view.

As shown in FIG. 1 , the gas detection system 1 is configured as a Lidardevice.

In the embodiments illustrated and described here, the gas detectionsystem 1 is operable to output the first output radiation 6 in theinfrared region of the electromagnetic spectrum, specifically at awavelength of between approximately 1.6506 pm and 1.6512 pm. However, inother embodiments the gas detection system 1 could be operable to outputthe first output radiation 6 in one or more regions of theelectromagnetic spectrum.

The first output radiation 6 comprises a continuous wave (CW) output andthe modulator 14 is operable to apply a first output random orquasi-random modulation (RM) (an example of first output modulation 16)to the first output radiation 6. In this arrangement, the gas detectionsystem 1 is a CWRM device.

As shown in FIG. 1 , the gas detection system 1 comprises a single laserdevice 4, and the emission wavelength 9 is continuously tuned within thewavelength spectrum 10, such that this single laser device 4 is used togenerate both the “on” wavelength(s) and the “off” wavelength(s). Thatis, in this embodiment the gas detection system 1 does not require aplurality of laser devices 4 to scan the wavelength spectrum 10. It willbe appreciated that in other embodiments, the gas detection system 1could comprise a plurality of laser devices 4, used to scan multiplewavelength spectra 10 or, in some embodiments, to use more than onelaser device 4 to scan within a particular wavelength spectrum 10 (e.g.using one laser device 4 to emit an “off” wavelength and another laserdevice 4 to emit an “on” wavelength).

In the embodiments illustrated and described here, the gas detectionsystem 1 includes a single optical system 26 operable to transmit thefirst output radiation 6 and to receive the scattered radiation 20. Theoptical system 26 may comprise a pair of prisms which may be driven,e.g. rotated, in manners to be described further below, to scan thelaser beam 20 across a system field of view.

The gas detection system 1 may be configured to be mountable to a framemember, a vehicle, an aerial vehicle, and/or an unmanned vehicle, anunmanned aerial vehicle, and/or a helicopter.

As shown in FIG. 1 , the gas detection system 1 includes one or moreoptical guide elements 28 configured to guide the received scatteredradiation 20 to the, or each, detector 22.

The laser device 4 is a tuneable laser device 4. In the embodimentillustrated in FIG. 1 , the first emission wavelength 9 of the laserdevice 4 is tuneable by adjusting, or modulating, the drive current ofthe laser device 4, which in this embodiment is controlled by thecontrol element 8 sending a drive current modulation 8 a to the laserdevice 4. It will be appreciated that in some embodiments, the laserdrive current could be provided directly from the control element 8, orvia ancillary drive circuitry.

FIGS. 2A-C are a series of diagrams illustrating how dual prisms, whichmay form part of the optical system 26 in the system of FIG. 1 , may beused to steer a laser beam. The prisms are generally wedge shaped andthe angle between their opposing major surfaces is referred to as the“wedge” angle.

Referring back to FIG. 1 , the laser device 4, the detector 22 and guideelements 28 together form a transceiver that transmits radiation to atarget location 18 and receives radiation from the target location viathe optical system 26. The transceiver is represented by block 200 inFIG. 2 . The optical system 26 may comprise a pair of prisms 221, 222,for example a Risley pair, which may be operated to cause a beam 201passing through them, optionally via a lens 223, to scan a field ofview.

In the following, examples using two prisms are described. However itshould be noted that the methods and systems described here may use orcomprise more than two prisms. For example equivalent effects to thoseachieved with two prisms may be achieved with more than two prisms, forexample multiple pairs of prisms, or with moving mirrors.

FIG. 2A shows two prisms aligned, and arranged to deflect the opticalbeam 201 in the +y direction with respect to the transceiver axis z. Thetransceiver axis is the axis about which the prisms are designed torotate and we will refer to both prisms being aligned as shown in FIG.2A being at an angle of 0 degrees and as having a relativeprism-to-prism angle of 0 degrees.

FIG. 2B shows two prisms aligned, arranged to displace but not deflectthe beam 210 in the +y direction. The displacement has caused theoptical beam 201 to travel along a path that is parallel to its originaldirection. We will refer to the first prism as being still at 0 degreesand the second prism as being at 180 degrees and the two prisms alignedopposite each other in this way as having a relative prism-to-prismangle, or relative prism angle in short, of 180 degrees.

FIG. 2C shows two prisms aligned, arranged to deflect the beam 201 inthe—y direction. While both these prisms have rotated now to 180 degreesthey have returned to a position where they again have a relativeprism-to-prism angle of 0 degrees.

By comparing FIGS. 2A and 2C it can be seen that if the prisms arerotated about the transceiver axis z without varying the relative anglebetween the prisms, the beam can be made to move in a circular path.

It will be appreciated that by rotating one or both of the prisms indifferent ways the beam may be deflected in angle in different ways toscan different fields of view. The field of view defined by this prismrotation and optical beam scanning is termed the system field of view(“SFOV”). The instantaneous field of view of the transceiver,represented by the area of the beam 201, is termed the transceiver fieldof view.

Thus there is provided here an optical scanner system, an example ofwhich is described further with reference to FIG. 10 , that translatesthe instantaneous narrow field of view of an optical transceiver(“TFOV”) around a larger system field of view (“SFOV”). The system maycomprise two identical or essentially identical optical prisms whichrotate in a controlled manner. Accordingly, some embodiments may includea Risley pair. The SFOV may be defined by the capability of a scanningsystem to translate, or move, the TFOV. It should be appreciated,however, that the scanning of the TFOV is not limited to the use of apair of prisms and other scanning mechanisms may be used.

FIGS. 3A and 3B show how the prisms 221, 222 may be incorporated into amechanical design. In this example the prisms are provided in a housing300 that contains rotary electric motors, one for each prism, andmagnetic encoders, one for each prism, to track the rotary motion of theprisms.

One function of the optical scanner system, to be described further withreference to FIG. 10 , may be to translate the TFOV around the SFOV inorder to build up an image. Note that for a coaxially aligned opticaltransceiver, TFOV is both the field of regard (pointing direction) ofthe outgoing of the collimated laser pulses, and the field of view ofthe detector. In other transceivers the TFOV might be one but not theother.

Further, in a method to be described in more detail below, the sensingmeasurements or image data may be used to determine the presence andlocation of excess gas in the field of view, e.g. gas present in aconcentration above a threshold value. An adjusted system field of viewmay then be determined in order to obtain sensing measurements at ahigher spatial resolution.

Another function of the optical scanner system may be to simply move theTFOV in order to reduce noise due to laser speckle (random spatialinterference patterns commonly observed when using a coherent lightsource). A static TFOV (and static target) is subject to high specklenoise, which impacts the metrological SNR of the sensor; moving the TFOVblurs out the speckle.

In the system of FIG. 1 a tuned radiation wavelength, for example laserlight, may be used to detect the gas. In that case the rate ofwavelength tuning may be chosen to be sufficiently fast that rapidmovement of the laser spot as a result of the scanning does not affectthe spectroscopic gas absorption measurements. For example, with thespot moving at a speed greater than 1 m/s across the system field ofview a tuning rate of 100 kHz or more may be used.

The optical scanner system may form part of a detection system sensorengine along with the optical transceiver. It may be controlled by motordrive circuitry in the system hardware (electronics), which mayinterface to a field programmable gate array “FPGA”.

Referring again to FIGS. 2A-C, in some embodiments the scannertranslates TFOV by rotating its prisms about the transceiver axis (z).By adjusting the relative prism-to-prism angle of the prisms, in otherwords the orientation of the “wedge” angles, TFOV can be moved to anypoint within SFOV. The angular pointing of SFOV (interchangeablyreferred to as the output beam vector) can be determined by thefollowing set of equations [Proc. of SPIE Vol. 6304, 630406, (2006) doi:10.1117/12.6790].

$\begin{bmatrix}k_{3x} \\k_{3y} \\k_{3z}\end{bmatrix} = {\begin{bmatrix}{\cos{\phi sin\theta}} \\{\sin{\phi sin\theta}} \\{\cos\theta}\end{bmatrix} = \begin{bmatrix}{{\beta\sin\alpha} + {\cos\phi^{\prime}\sin{\alpha\left\lbrack {\sqrt{1 - n^{2} + {\gamma^{2}\left( \phi^{\prime} \right)}} - {\gamma\left( \phi^{\prime} \right)}} \right\rbrack}}} \\{\sin\phi^{\prime}\sin{\alpha\left\lbrack {\sqrt{1 - n^{2} + {\gamma^{2}\left( \phi^{\prime} \right)}} - {\gamma\left( \phi^{\prime} \right)}} \right\rbrack}} \\{\left( {1 + {\beta\cos\alpha}} \right) + {\cos{\alpha\left\lbrack {\sqrt{1 - n^{2} + {\gamma^{2}\left( \phi^{\prime} \right)}} - {\gamma\left( \phi^{\prime} \right)}} \right\rbrack}}}\end{bmatrix}}$$\beta = {\sqrt{n^{2} - {\sin^{2}\alpha}} - {\cos\alpha}}$γ(ϕ^(′)) = cos α + β(cos²α + cos ϕ^(′)sin²α)

In the above, the 1st prism in the pair is at 0° rotation (as in FIG.2A), and the variables are defined as:

-   -   k₃ is the vector of the optical beam at the output by the Risley        pair, assuming an input beam parallel to the z axis (k₁=[0, 0,        1]).    -   θ is the polar angle of the output beam    -   ϕ is the azimuthal angle of the output beam    -   α is the prism wedge angle    -   ϕ′ is the relative prism-to-prism angle between the prisms about        the optical axis (ϕ′=0° in FIGS. 2A and 2C, ϕ′=180° in FIG. 2B)    -   n is the refractive index of the prisms.

It should be appreciated that the spatial patterns of beam vector dependupon the mode of operation of the scanner. There are four primaryconsiderations when designing scan patterns, one or more of which may beused in any combination depending on the particular requirements of asystem:

-   -   Speckle noise: minimized by maintaining high beam speed across        entire scan pattern.    -   Averaging: Noise can be reduced by averaging; thus high point        density areas will give lower noise. Again, Speckle noise is        maximum for low beam speed thus areas of low beam speed could        have higher point density to compensate    -   Resolution: resolution is reduced at higher beam speed (motion        blur) and increased at higher point density. Thus, an effective        scan pattern should have low beam speed where point density is        high (and vice versa) since the alternative—high speed and high        point density—risks blurring points together.    -   Mechanical: mechanical simplicity and consistency of the wedge        motor motion. Effective scan patterns should have no sudden        changes in direction or rapid accelerations that the system        hardware cannot handle.

Some of the methods described here are particularly concerned with theresolution of scanning for the purpose of reliably performing gassensing measurements.

In general the scanning may operate in one or more of 4 modes ofoperation:

Mode 1: Asynchronous Full FOV

Running the two wedges asynchronously at fixed speeds.

Wedge speeds can be chosen to be significantly different, for examplethe speed of one can be as little as a tenth of the speed of another,(0.447 Hz vs 1 Hz in a specific example) since this prevents very lowspeeds (^(˜)0 m/s) at the center of the pattern.

Simple scan patterns produce a similar point density distribution. Veryhigh density in the center, lower toward the edges.

However, less than 20% of scan points are in the central 5 m radius ofthe pattern. Redistributing these points to the edges has minimal effecton average point density.

Thus the inhomogeneous point density is not a serious issue.

FIGS. 4A and 4B show two examples of beam angular scanning patterns thatcorrespond to Mode 1 described above, e.g. generated by a prism pairrotating asynchronously. In each case lighter shades indicate a fasterscan speed. FIG. 4A shows a pattern that may be achieved with prismsrotating in the same direction. FIG. 4B shows a pattern that may beachieved with prisms rotating in opposite directions. In each case therotation is asynchronous, i.e. the prisms are rotating at differentrates.

With different rotation rates a two-prism scan pattern may fill in acircle with, for example, a maximum deflection equal to twice a singleprism's deflection.

Wedges rotating in the same direction maximize beam velocity at edges ofscan, while wedges rotating in the opposite direction minimize beamvelocity at edges of scan.

For full FOV imaging the beam speed may be maximized where point densityis low.

-   -   Low point density→high noise    -   High beam speed→low (speckle) noise

Thus wedges rotating in same direction produce an advantageously evensensitivity level across the whole FOV.

On the other hand, spinning the wedges in opposite directions achieveshigh SNR in the center and low on the edges. However, some embodimentsmay use scan mode 2 to zoom in on the point of interest (with relativewedge phase control).

Mode 2: Synchronous Circle

When only a small spatial region needs to be scanned, but speckle mustbe reduced, a synchronous circle pattern may be used. A synchronouscircle pattern may be understood as one that is generated by rotatingthe prisms synchronously together with each other while fixing therelative prism-to-prism angle in a set orientation. This fixedorientation determines the diameter of the SFOV circle.

This scan pattern can then be generated by synchronizing the two prism'srotation such that there is no relative rotation between thewedges—i.e., a fixed relative prism-to-prism angle is maintained.Rotating the two prisms at the same rate will make the output opticalbeam angle scan in a circle with the rotating prisms. As described inFIG. 2 , it is possible to select the angular radius of this circle astwo times the single prism deflection angle when the relative prismangle is 0 degrees, as shown in FIGS. 2A and 2C, or to select theangular radius of the circle as zero when the relative prism angle is180 degrees, as shown in FIG. 2B. Varying the fixed relativeprism-to-prism angle will change the angular radius of the outputoptical beam scan circle to any value between these two limits. Here theterm “prism deflection angle” is used to refer to the angle ofdeflection of the optical beam from the z axis due to the presence ofthe prism.

FIGS. 5A and 5B illustrate how optical beam circular scan patterns asdescribed in Mode 2 may be achieved using a synchronous pair of prisms,i.e. rotating in the same direction and at the same rate. With equalrotation rates a two-prism scan pattern forms a simple circle with adeflection that depends on the fixed relative prism-to-prism angle.

FIG. 5A shows different circle scans with different prism-to-prismangles. The example corresponds to a prism pair where one prism deflectsan optical beam passing through it by 10 degrees and so two prismsaligned together at 0 degrees relative prism angle deflects an opticalbeam by 20 degrees. Two prisms with relative prism-to-prism angle of 120degrees scan a circle with 10 degrees radius, so produce a 2× zoom, andtwo prisms with relative prism angle of 150 degrees scan a circle with 5degrees radius and produce a 4× zoom. FIG. 5B is a corresponding graphof relative prism-to-prism angle versus resulting scanned circle size.It will be appreciated from FIGS. 5A and 5B that the TFOV may betranslated around a series of circles by changing the fixed relativeprism-to-prism angle, for example from one rotation to the next or atdifferent intervals.

Mode 3: Synchronous Zoom

Mode 2 operation creates an optical zoom effect by varying the relativeprism-to-prism angle. However open circles like those generated this wayare of limited use in optical beam scanning. What is more generallyuseful is to create scans that completely fill in these smaller zoomedcircles without going outside the smaller outer radius.

This may be achieved in various ways, some of which will now bedescribed.

One option is to apply a variable phase shift to the relativeprism-to-prism angle of the two prisms. Applying the variable phaseshift may involve rotating the two prisms at the same nominal, oraverage, rate which may be the same as in Mode 2, and modulating therotation rate of one or both of the prisms in a periodic manner to varythe relative orientation of the prisms, or prism to prism angle. Thismodulation may include a maximum amplitude so as to limit the range overwhich the relative prism-to-prism angle can be varied whilst stillallowing the scanning of a small circular spatial region with morecomplete filling in. For example, and building on the previous Mode 2description, to scan with a zoom factor of 2× compared to asynchronousprism rotation but with a more filled-in field of view the relativeprism-to-prism angle would be varied from 180° by plus and minus 60°.This could be achieved by modulating the rotation rate of one of the twoprisms with a sinusoidal function having a given period to result in arelative rotation angle that ranges from 120° to 240° orϕ′=180°*(1+0.333 sin(t/period).

In this example, one prism is rotated at a constant rate and the rate ofrotation of the other prism is varied periodically. It will beappreciated that the same effect can be achieved by periodically varyingthe rate of rotation of both prisms rather than one rotating at aconstant rate, for example by varying a relative rate of rotation of oneprism with respect to the other. Whether or not one prism rotates at aconstant rate, the average or nominal rate of rotation of the two prismsmay be the same.

The FOV can be adjusted by changing the amplitude of the sine function,0.333 in the example above. In some embodiments, one may also use a sawtooth function or similar instead of a sine function. Again, for a fixedsynchronous prism rotation rate the speed of the angles scanned ismaximized at the high angles at the edges of the pattern.

For a lidar system with a fixed data acquisition rate a smaller opticalfield of view leads directly to a larger number of data points in anyparticular spatial area and therefore a corresponding increase inspatial resolution. Mode 3 operation with a reduction in the size of thefield of view is therefore higher spatial resolution compared withmode 1. However, beam scanning speed is also lower for the same prismrotation rates, which may impact speckle noise.

FIGS. 6A-C illustrate an example of a zoom pattern of the type describedin mode 3 where the effects of superimposing an oscillation of therelative prism-to-prism angle on top of a synchronous rate of rotationof a prism pair are used to achieve a reduced size scanning pattern. Thesize of the pattern, i.e. the SFOV, can be adjusted by varying themaximum relative prism-to-prism angle about 180°. This example has thatangle change by plus and minus 60° from 120° to 240° and the scanpattern fills in a circle of 10° radius and so has a zoom factor of 2compared to the asynchronous result in FIG. 4 . This example varies theprism-to-prism angle as a sinusoid in time with a period 1.116 times theaverage or nominal rate of rotation. The period of modulation of therelative rate of rotation of one prism with respect to the other may bekept close to 1 to minimise the variation in scanning speed of the laserbeam. We see for instance that for sinusoidal modulation a ratio ofmodulation period to the period of the average rate of rotation ofbetween 0.6 and 2.0 (i.e. 60% to 200%) times keeps the scanning speed to<50% variation.

FIG. 6 illustrates a zoom of 2× where the radius of the field of view is10 degrees and not the 20 degrees of the asynchronous mode 1. This zoomfactor is adjustable by changing the maximum amplitude of the relativeprism-to-prism angle. FIG. 6A shows the variation in the two prismangles over time, FIG. 6B shows the build-up of the fill of the circlein time with the scanned patterns obtained after 1, 5, 20 and 60seconds, and FIG. 6C shows the resulting scanning speed of the opticalbeam over the same 60 seconds.

Mode 4: Synchronous Spiral

Another similar approach to mode 3 is using synchronized prisms with asweeping prism-to-prism angle to draw a spiral trajectory. Spiralpatterns are achieved whenever the period of oscillation of theprism-to-prism relative rotation angle is many times longer than theperiod of the combined synchronous rotation. In other words, the periodof the modulation of one of the prism's rotation rates is many timeslonger than the period of that prism's unmodulated rotation. This mayalso be understood as meaning that the modulation rate—i.e., the rate ofthe modulation of one of the prism's rotation rate—is many times slowerthan that prism's unmodulated rotation rate. We see for instance thatfor sinusoidal modulation a ratio of modulation period to the period ofthe average rate of rotation of greater than 6 (i.e. 600%) timesproduced a spiral scan pattern.

Spiral patterns can therefore be combined with mode 3 to scan with zoomover a smaller target area. However the scanning speed drops and specklewill become large if the rotation rate in the center of the spiral isnot increased.

FIGS. 7A-C are illustrations of a spiral scan pattern as described inmode 4. This example is in every way similar to the example shown inFIGS. 6A-C with the single change that the oscillation rate of therelative prism-to-prism angle is now around 20× slower. In this way thereduced size scanning pattern has a spiral shape and a more uniformpoint density across the SFOV. It does however have a very slow scanningspeed of the optical beam in the center of the SFOV.

As with FIGS. 6A-C the relative prism-to-prism angle is varied from 120to 240 degrees as a sinusoid in time to create a pattern with a zoomfactor of 2 compared to asynchronous prism rotation. This example showsthe spiral pattern that results with a period of relative prism anglechange that is 19.83 times the synchronous rate of prism rotation.

A spiral pattern can allow for uniform sampling of the SFOV and uniformspeckle reduction, provided that the speed of the prism rotation iscontrolled throughout the scan to maintain a constant spot translationspeed at the target: The prisms rotate slowly at the outside of thespiral pattern, and then speed up as the pattern approaches the center.

FIGS. 8A-C show a spiral pattern very similar to FIGS. 7A-C but with theadditional change that the rate of the synchronous rotation of the prismpair is increased as the pattern spirals into the center so that theminimum scanning speed of the optical beam is substantially increased.

As described with mode 3, limiting the range of this relative prismangle allows a variable optical zoom, and as with mode 3, by reducingthe outer radius of the pattern whilst maintaining the number of sampledpoints per scan and spot translation speed at the target, mode 4operation allows for higher spatial resolution compared with mode 1.

Point density uniformity of a scan created by mode 3 or mode 4 is afunction of how much successive sweeps of the scan overlap spatially.This overlap can be high and large gaps in the scan pattern can occur ifthe ratios of synchronous and relative prism rotations are regularfactors. In FIGS. 6A-C, 7A-C, and 8A-C these ratios have been chosenparticularly to minimize this overlap and create a more uniform patternfill. This uniformity may be further increased by adding random orregular timing variations to parts of the interleaved trajectories. Forinstance is also possible to further increase sampling coverage (andspeckle reduction) by adding additional dither motion to the spiralpattern.

To mitigate effects of Hardware/Mechanical limitations on prism rotationacceleration, where the trajectory reverses or change direction,patterns may be used in which there is no sudden reversal of directionat high speed.

FIGS. 9A-C show gas lidar images acquired with different system fieldsof view, varied using the techniques described in Mode 4 above. In eachrow, from left to right, the first image shows the lidar scatter returnintensity and therefore contains information relating to solidstructures in the field of view. The second image shows gasconcentration pathlength, determined for example using any method knownin the art, with the scale in ppm.m (parts per million gas times opticalpathlength) shown adjacent. The third image shows gas concentrationpathlength overlaid on reflected intensity. This enables the source ofthe gas to be identified relative to the solid structures. To obtain theimages a pipe with a constant flow of methane from a nozzle was held ina workbench so the gas was directed up into the air. This was imagedwith a lidar gas detection system at various zoom levels. Notably theimages of FIGS. 9A-C may be obtained using one laser. In other words,the same laser may be used to form an image of the solid structures aswell as to sense gas in the atmosphere around the solid structures.Further, this information may be obtained in the same scan of the systemfield of view. Therefore any of the methods described here may compriseperforming a spatially scanned lidar measurement of a) ranges tosurfaces and b) gas concentration pathlength, wherein these are measuredover the same distances to the same surfaces across a field of view.Then the position of any cloud of excess gas above the background inthat field of view may be determined and the spatial scan's field ofview, i.e. the system field of view, may be adjusted to improve theresolution of the measurement of the cloud of excess gas. Importantlythe adjustment may be based on the gas sensing measurements alone.

FIG. 10 is a schematic diagram of a gas lidar scanning system that maybe used to implement any of the methods described here. The system ofFIG. 10 may be incorporated into the gas lidar detection system of FIG.1 .

The scanning system comprises a transceiver 1010 which may for examplecomprise the laser device 4, the detector 22 and guide elements 28 ofthe system of FIG. 1 . The system of FIG. 10 further comprises anoptical system 1012 corresponding to optical system 26 of FIG. 1 , shownhere to comprise a pair of prisms as described above. The optical system1012 may comprise additional components such as but not limited tolenses as will be understood by those skilled in the art.

The transceiver 1010 may be mechanically fixed with respect to theoptical system 1012 and controlled by pan and tilt stages 1020, 1030 topoint the transceiver 1010 at a target area for transmission andreception of laser radiation. The pan and tilt stages 1020, 1030 may bedriven by respective motors 1021, 1031 under the control of pan/tiltcontroller 1023. Position signals from the pan and tilt stages 1020,1030 may be generated by respective encoders 1022, 1032 and fed back tothe pan/tilt controller 1023.

The optical system 1012 may be operated as described above to scan thelaser over the target area within a system field of view. This may bedefined by the rotation of the prisms as described above to achieve adesired coverage and/or resolution. For this purpose a respective motor1013, 1014 is provided to drive each prism whereby the two prisms may berotated together (synchronously) or with respect to each other. Positionsignals from the prisms may be generated by respective encoders 1015,1016 and fed back to a scanning control system 1040. The scanningcontrol system 1040 may be implemented in software and may comprisemotor drive 1041 configured to send drive signals to motors 1013, 1014,an image processor 1042 whose function is described further below, andencoder processor 1043 configured to process signals from the encoders1015, 1016.

The image processor 1042 is configured to send and receive signals froma system controller 1050. The system controller 1050 is also configuredto send and receive signals from an inertial measurement unit 1060.

The system controller 1050 and the pan/tilt controller 1023 may formpart of the control element 8 of FIG. 1 .

The system of FIG. 10 may operate as a lidar system that simultaneouslywith one laser determines the distance to a remote surface and the gasabsorption of laser light over the distance to the surface, for exampleas shown in FIGS. 9A-C. The system may comprise transceiver configuredto transmit and receive the laser light that is used for range and gasmeasurement, as described with reference to FIG. 1 . As shown in FIG. 10a laser beam scanner is provided comprising multiple encoders 1015, 1016configured to indicate the direction of the spatially scanned laserlight that is used for range and gas measurement, and a set ofprocessors 1043, 1040 configured to process data from the multipleencoders in the scanner and generate spatially registered range and gasconcentration data.

The system of FIG. 10 may be configured to implement a method ofoperating a lidar system for detection of a gas, now described withreference to FIG. 11 .

At operation 111 in FIG. 11 , spatially scanned sensing measurements ofthe gas are performed across a system field of view. This may be asystem field of view used for regular monitoring of possible gas leaksfor example. In order to achieve this the lidar may be scanned usingrotating prisms as described above, for instance as in Mode 1 or Mode 3or Mode 4, and the system field of view may therefore be defined by thescan pattern. From these measurements it is possible to measure gasconcentration pathlength across a field of view where there is apotential for a gas leak.

At operation 112, the lidar sensing measurements are analyzed todetermine the presence and location of any excess gas in the field ofview, for example determined by the gas concentration pathlength beingabove a certain threshold and optionally over a predetermined area. Inother words, areas in the system field of view with gas concentrationabove background and/or noise levels, for example represented by athreshold, may be identified. This may be achieved by forming a gasconcentration image for example by System Controller 1050, to identifyareas that indicate an excess gas cloud (e.g. above threshold area)and/or a gas leak (smaller area, above threshold concentrationpathlength).

Following the analysis in operation 112, based on the determinedlocation, an adjusted system field of view is determined and thenspatially scanned sensing measurements of the gas are performed acrossthe adjusted system field of view to obtain sensing measurements athigher spatial resolution. In the method shown in FIG. 11 this is donein multiple operations and used to verify measurements of the leak ratecalculation.

Broadly, the method may comprise calculating a gas flow rate from thesensing measurements after each spatial scan at operation 113, comparingsuccessive flow rate measurements at operations 114 and 116, andrepeating the performing and analyzing until the flow rate measurementsconverge on a consistent value, e.g. are within a predetermined margin.

Thus at operation 113 in FIG. 11 a gas flow rate is determined. This maybe done in a manner known in the art, for example using the gasconcentration image of the kind shown in FIGS. 9A ii, 9B ii, and 9C ii,the range image as shown in FIGS. 9A i, 9B i, and 9 c i, and the windspeed.

In operations 114 and 116 it is determined whether the gas leak rate isequivalent, e.g. within a predetermined range of, a previouslydetermined leak rate. If it is not, as determined at operation 114, thisfurther confirms a likely leak that should be investigated more closely.Therefore at operation 115 the system field of view size is adjusted,for example as shown in FIGS. 9A-C to improve the resolution of themeasurement of excess gas. Additionally or alternatively the pan andtilt of the system may be adjusted so that areas of excess gas are morecompletely imaged.

Operations 114 and 115 may be repeated until the measurement of gas flowrate is consistent, as determined at operation 116 where gas flow ratecalculated at operation 113 is equivalent to the previously calculatedflow rate. At this point a leak may be reported at operation 117, wherethe gas concentration image and the gas flow rate may be recorded.

In some embodiments of the present invention, the methods and/oralgorithms described herein may be implemented by suitable configurationof a computing system in an existing device. In other embodiments,measurement results may be received at a computing system from a lidardevice and processed to determine gas concentration path.

A system as described herein may comprise a computing system or device.Such a system or device may comprise one or more processors which may bemicroprocessors, controllers or any other suitable type of processorsfor processing computer executable instructions to control the operationof the device in order to gather and record routing information. In someexamples, for example where a system on a chip architecture is used, theprocessors may include one or more fixed function blocks (also referredto as accelerators) which implement a part of the method in hardware(rather than software or firmware). Platform software comprising anoperating system or any other suitable platform software may be providedat the computing-based device to enable application software to beexecuted on the device.

Various functions described herein can be implemented in hardware,software, or any combination thereof. If implemented in software, thefunctions can be stored on or transmitted over as one or moreinstructions or code on a computer readable medium. Computer-readablemedia may be non-transitory and include, for example, computer readablestorage media. Computer readable storage media may include volatile ornon-volatile, removable or non-removable media implemented in any methodor technology for storage of information such as computer readableinstructions, data structures, program modules or other data. A computerreadable storage medium can be any available storage medium that may beaccessed by a computer. By way of example, and not limitation, such acomputer readable storage medium may comprise a RAM, a ROM, an EEPROM, aflash memory or other memory device, a CD-ROM or other optical discstorage, a magnetic disc storage or other magnetic storage device, orany other medium that can be used to carry or store desired program codein the form of instructions or data structures and that can be accessedby a computer. A computer readable storage medium and/or computerreadable storage media, as used herein, is not to be construed as beingtransitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Alternatively, or in addition, the functionality described herein can beperformed, at least in part, by one or more hardware logic components.For example, and without limitation, hardware logic components that canbe used may include Field-programmable Gate Arrays (“FPGAs”),Program-specific Integrated Circuits (“ASICs”), Program-specificStandard Products (“ASSPs”), System-on-a-chip systems (“SOCs”). ComplexProgrammable Logic Devices (“CPLDs”), etc.

Although illustrated as a single system, it is to be understood that thecomputing device may be a distributed system. Thus, for instance,several devices may be in communication by way of a network connectionand may collectively perform tasks described as being performed by thecomputing device.

It will be appreciated that a computing device as described here may belocated remotely and accessed via a network or other communication link(for example using a communication interface).

The term “computer” is used herein to refer to any device withprocessing capability such that it can execute instructions. Thoseskilled in the art will realize that such processing capabilities areincorporated into many different devices and therefore the term‘computer’ “computer” includes PCs, servers, mobile telephones, personaldigital assistants and many other devices.

As used herein, the terms “component” and “system” are intended toencompass computer-readable data storage that is configured withcomputer-executable instructions that cause certain functionality to beperformed when executed by a processor. The computer-executableinstructions may include a routine, a function, or the like. It is alsoto be understood that a component or system may be localized on a singledevice or distributed across several devices.

While the methods are shown and described herein may be series of actsthat are performed in a particular sequence, it is to be understood andappreciated that the methods are not limited by the order of thesequence. For example, some acts can occur in a different order thanwhat is described herein. In addition, an act can occur concurrentlywith another act. Further, in some instances, not all acts may berequired to implement a method described herein.

It will be understood that the above description is given by way ofexample only and that various modifications may be made by those skilledin the art. What has been described above includes examples of one ormore embodiments. It is, of course, not possible to describe everyconceivable modification and alteration of the above devices or methodsfor purposes of describing the aforementioned embodiments, but one ofordinary skill in the art can recognize that many further modificationsand permutations of various embodiments are possible. Accordingly, whileone or more embodiments of the invention have been illustrated anddescribed in detail in the drawings and foregoing description, the sameis to be considered as illustrative and not restrictive in character, itbeing understood that only certain embodiments have been shown anddescribed and that all changes and modifications that come within thespirit of the inventions are desired to be protected.

One or more of the features of the appended claims may be implemented inany combination in order to implement an embodiment of the invention.

It is not a requirement of any embodiment of the invention, unlessotherwise stated, to perform the steps of the method in a particularorder.

The invention claimed is:
 1. A method of operating a lidar system fordetection of a gas, wherein the system comprises an optical transceiverfor transmitting and receiving optical radiation, the method comprising:performing spatially scanned sensing measurements of the gas across asystem field of view; analyzing the sensing measurements to determinethe presence and location of an excess of the gas in the system field ofview; based on the determined location, determining an adjusted systemfield of view and performing spatially scanned sensing measurements ofthe gas across the adjusted system field of view to obtain sensingmeasurements at higher spatial resolution; calculating a gas flow ratefrom the sensing measurements after each spatial scan; comparingsuccessive flow rate calculations; and repeating the performing andanalyzing until the flow rate measurements converge on a consistentvalue.
 2. The method of claim 1, wherein each of the performingspatially scanned sensing measurements of the gas comprises performing aspatially scanned lidar measurement of ranges to surfaces and gasconcentration pathlength, and wherein the ranges and pathlengths aremeasured over the same distances to the same surfaces across a field ofview.
 3. The method of claim 1, wherein each of the performing spatiallyscanned sensing measurements of the gas comprises translating theoptical transceiver field of view (TFOV) around a larger system field ofview (SFOV) in one or more patterns to detect gas within the largersystem field of view, and wherein determining an adjusted field of viewcomprises determining an adjusted pattern for the translation of thetransceiver field of view.
 4. The method of claim 3, wherein the lidarsystem comprises two prisms for guiding the path of optical radiationand the method comprises rotating the prisms asynchronously at fixedspeeds to translate the TFOV.
 5. The method of claim 3, wherein thelidar system comprises two prisms for guiding the path of opticalradiation and the method comprises rotating the two prisms relative toeach other to a predetermined relative orientation and rotating the twoprisms at the same rate to translate the TFOV around a circular path. 6.A method of operating a lidar system for detection of a gas, wherein thesystem comprises an optical transceiver for transmitting and receivingoptical radiation, the method comprising: performing spatially scannedsensing measurements of the gas across a system field of view; analyzingthe sensing measurements to determine the presence and location of anexcess of the gas in the system field of view; and based on thedetermined location, determining an adjusted system field of view andperforming spatially scanned sensing measurements of the gas across theadjusted system field of view to obtain sensing measurements at higherspatial resolution, wherein each of the performing spatially scannedsensing measurements of the gas comprises translating the opticaltransceiver field of view (TFOV) around a larger system field of view(SFOV) in one or more patterns to detect gas within the larger systemfield of view, wherein determining the adjusted field of view comprisesdetermining an adjusted pattern for the translation of the transceiverfield of view, and wherein the lidar system comprises two prisms forguiding the path of optical radiation and the method comprises rotatingthe two prisms at the same average rate and varying the relative anglebetween the prisms by modulating the rate of rotation of one or both ofthe two prisms to change the system field of view.
 7. The method ofclaim 6, wherein the relative angle is varied by up to 180 degrees. 8.The method of claim 6, wherein the modulation varies the rotation rateperiodically.
 9. The method of claim 8, wherein a ratio of the period ofmodulation to the period of the average rate of rotation is between 0.6and
 2. 10. A method of operating a lidar system for detection of a gas,wherein the system comprises an optical transceiver for transmitting andreceiving optical radiation, the method comprising: performing spatiallyscanned sensing measurements of the gas across a system field of view;analyzing the sensing measurements to determine the presence andlocation of an excess of the gas in the system field of view; and basedon the determined location, determining an adjusted system field of viewand performing spatially scanned sensing measurements of the gas acrossthe adjusted system field of view to obtain sensing measurements athigher spatial resolution, wherein each of the performing spatiallyscanned sensing measurements of the gas comprises translating theoptical transceiver field of view (TFOV) around a larger system field ofview (SFOV) in one or more patterns to detect gas within the largersystem field of view, wherein determining the adjusted field of viewcomprises determining an adjusted pattern for the translation of thetransceiver field of view, and wherein the lidar system comprises twoprisms for guiding the path of optical radiation and the methodcomprises rotating the two prisms at the same average rate, varying therelative angle between the prisms to change the field of view andvarying the average rotation rate to change the scanning speed.
 11. Themethod of claim 1, wherein the optical wavelength is tuned to detect thegas.
 12. The method of claim 11, wherein the rate of tuning is chosen inrelation to the rate of movement of the TFOV to ensure that the movementof the TFOV does not affect the detection of the gas.
 13. The method ofclaim 11, wherein the rate of wavelength tuning is at least 100 kHz. 14.The method of claim 3, wherein the speed of movement of the TFOV ischosen to be sufficiently high to reduce speckle noise.